Contrary to what its ‘City That Never Sleeps’ nickname may have you believe, people do actually sleep in New York. Surrounded by brownstones and high rises, the surface level operations for the TBM mining Phase One of the Second Avenue Subway fall quiet every night at 10. Underground is a different story. A joint venture of Skanska, Schiavone and Shea (S3 Tunnel Constructors) mines 24 hours a day, five days a week.
“We can’t get deliveries and we can’t hoist anything with a crane from 10pm to 7am because it’s noisy. You almost have to plan with a military precision and have it set up. One has to foresee what will be needed precisely in order to coordinate material deliveries,” says Julio Martinez, project engineer, Schiavone Construction Company.
In addition to a restriction on the hours for deliveries, spoil removal and other work in the launch box, there is a lack of space in the construction area that complicates these cautiously organized deliveries. “We can’t stack or store large volumes of materials. We have to truck them in and out of the tunnel as we receive them. It’s a constant chess game to get materials into the tunnel and keep everything in a productive manner,” adds Chris Cosenzo, tunnel superintendent, Schiavone.
With the first of two drives completed and the second to start this spring, the crews may be adjusting to the challenges of the operations, but they are also dealing with ground conditions arising from the shallow depths of the tunnel that have required ground freezing and slowed the pace of tunneling in general.
Challenges above ground
Proposals to build a north-south subway line along Second Avenue date back to 1929, and work has actually been undertaken several times in the past 70 years. Two elevated lines on Second and Third Avenues were demolished in the 1940s and ‘50s, and tunnel segments were built following a plan introduced in the 1960s. However, construction was suspended in the 1970s for financial reasons.
A study launched in 1995 led to the current plan to build a two-track line along Second Avenue from 125th Street to the Financial District in Lower Manhattan, with construction divided into four phases.
Phase One will connect 105th Street and Second Avenue to existing services at 63rd Street and Third Avenue, with new stations at 96th, 86th and 72nd Streets. Although the full length of the Second Avenue line won’t be completed, when Phase One finishes, the subway service is projected to carry more than 200,000 weekday riders.
The S3 contract involves new tunnels between 92nd and 63rd Streets, and excavation of the TBM launch box and access shafts at 69th and 72nd Streets. These shafts will be used for constructing the 72nd Street station, which is part of a different contract awarded to a joint venture of Schiavone, Shea and Kiewit (SSK Constructors) in October 2010. And rounding out Phase One, connection to existing tunnels from 99th to 105th Streets is being undertaken in separate contracts (Figure 1). A joint venture of DMJM Harris and Arup is providing design and engineering services, and Wang Technology is providing instrumentation and monitoring services under contract to S3.
Right in the very center of Second Avenue from 92nd to 95th Streets is the TBM launch box. Tunnelling started at 92nd Street to make use of the Manhattan Schist, which continues to the south (Figure 2). The tunnels range from 60 to 100ft (18.3 to 30.5m) depth from the surface, and each bore has a diameter of 22ft (6.7m) with an 18ft (5.5m) rock pillar separation between them. TBM excavation for the 7,215ft (2.3km) western bore finished this February at 65th Street. By April the TBM is expected to start the 7,793ft (2.4km) eastern bore, which will make a tight, westerly compound curve to hole-through into the existing 63rd Street Station.
The cut and cover excavation for the launch box measures roughly 810ft (247m) going north to south, approximately half in rock at the southern end and half in soil at the northern end. In the rock cut, the width is around 66ft (20m), and in the soil cut, the width is 58ft (17.7m). The box walls consist of two different types of construction, secant walls at the southern half and slurry walls at the northern half. The slurry walls at the middle portion of the box are founded within rock. Northward from there, the slurry walls are founded within the soil with approximately 40ft (12m) of toe below the final excavation limits. From the southern end to the middle portion of the box, the walls are constructed with secant piles founded within the rock.
New York’s Metropolitan Transportation Authority Capital Construction (MTACC) awarded the contract in March 2007, but it wasn’t until early 2010 that the contractor was ready to do underground work. “We had to perform a significant amount of utility and other preparatory work in order to build the walls of the box and that took about a good year, year and a half, ” says Martinez. “And then we had to install decking on top of the future excavation since traffic still had to be able to transverse above us while we excavated within the box.”
S3 elected to use one TBM for both bores to save space in the launch box and to avoid the logistics of doubling the electricity, water, ventilation and other services required. The JV is using a reconditioned open-face main beam gripper machine with 44 cutters, 17in in diameter, from Shea’s fleet—a decision that saved time in a tight schedule. Working with Herrenknecht, it took roughly five to six months to rebuild the machine.
“We took advantage of it when we bid this work,” Martinez explains. “And that’s why we’re the low bidder. Also we have experience with this machine on three other projects, thus we have very intimate knowledge of it. In addition, we do have the know-how and the facilities to refurbish the TBM ourselves.”
Selected stretches of the tunnels between station caverns will receive a 13.5in (343mm) thick fiber reinforced concrete lining. MTACC let the contract with two options including the use of a shielded TBM with precast concrete lining. “We made the choice to go with the gripper machine because there are gaps in the lining, and the space to have the pre cast segments wasn’t available,” he says. Noting again: “In addition the timely preparedness of the TBM was a critical schedule item.”
With the 10pm to 7am noise restriction, it is imperative there is always enough room in the launch box to store spoil. Muck trains bring material to a hopper in the launch box; from there a belt brings the muck to the northern most part of the box for storage in a bin. Material from the muck bin is loaded into a vertical belt and taken to the surface, where it is stored in a truck hopper. During the approved hours, trucks are loaded on site. Otherwise, material is stored. The hopper has a 90t capacity.
Major access to the launch box is on the southern most part, on the east side. After the TBM was assembled to full function and tested at Schiavone’s shop in New Jersey, it was disassembled and shipped to the project site. With the west bore being excavated first, everything was lowered down the shaft in reverse order and moved to the back of the launch box. Down below, another crane assembled the machine. Then the contractor actually slid the machine from east to west and advanced to the face.
“Assembly of a machine of this size in the area we have—it is challenging, but it just goes back to being able to plan your moves very cautiously and very well thought out,” Cosenzo says.
Challenges below
MTACC surveyed 225 buildings on Second Avenue that could be affected by tunneling work, and 51 buildings were deemed ‘too fragile’ in autumn 2010. These are primarily located near the TBM launch box, and are four to five-story brick buildings built in the 1800s with shallow foundations, supported on soil or timber piles.
Martinez describes them as basically floating on the soil, which is a mix of sand, silt and clays. Only prior to the construction of these buildings, this part of Manhattan was shoreline, which was then reclaimed to amplify the city’s limits. “This fill was not controlled. It wasn’t compacted as fill is placed in modern times. So whatever soil you move, it’s actually going to react much quicker,” he says.
MTACC initially predicted a progress of 50ft (15m) per day, and T&T reported in November 2010, TBM excavation was progressing at 40ft (12m) per day. Besides buildings there are utilities overhead— between 2 and 40ft away as the tunnel’s elevation changes through the alignment. Lengths and feed pressures on drilling and probing have to be less aggressive. “We’ve had to do more probes than what we normally would have with a machine at a deeper depth, which increased the mining cycle a little bit,” Cosenzo says.
As T&T went to press, the TBM had completed the western bore and the contractor was removing the four segments of the cutterhead around the perimeter, the roof supports and the side supports. The machine will be fitted with transport dollies and towed back to the launch box via the use of locomotives.
“What this is allowing us to do is, we’re bringing the machine back fully assembled with all the hydraulics and electrics still hooked up…When we come back and hit the launch box, there won’t be much reassembly except for the shields and the cutter head segments,” Consenzo explains.
He estimates tunneling will start in the eastern bore before the end of March, and should take around six to 10 months to breakthrough at the 63rd Street Station. Once the TBM is out of the western bore, select stretches of tunnel will be treated with a PVC membrane prior to receiving the lining.
There is an area approximately 147ft (44.8m) long at the portal of the east bore (around 91st Street) where Moretrench of New Jersey is providing ground freezing services. While doing secondary support for the starter tunnels S3 encountered soil and water.
“We notified the owner and performed exploratory borings. In the process we did find that part of the alignment, the crown, was in soil-like material. In light of these findings the owner prudently elected to freeze this stretch of the alignment,” Martinez explains.
Approximately 110 freeze pipes of 3in (76.2mm) internal diameter have been installed around the tunnel alignment at depths of approximately 75ft (23m), at varied angles.
“We had to angle as much as 30 degrees off of the vertical in order to avoid water lines, gas lines, all the utilities,” says Kenneth Wigg, senior engineer with Moretrench.
And the placement of freeze pipes created another problem, he explains. “We couldn’t get exactly where we wanted to be. So we had to really look at the placement of the freeze pipes, and in some places we had to add freeze pipes, to get those pockets we couldn’t reach because of the utilities.”
The freeze started in January and uses calcium chloride brine. As the machine bores the east tunnel a thin interliner will be installed to seal off and support the tunnel crown prior to the thaw.
Wigg says it typically takes as long to thaw as the systems was running. When he spoke to T&T, it had been close to nine weeks, in counting.
At the end of the 7,793ft (2.4km) east bore, the TBM makes a tight compound curve to meet up with existing tunnels and train services at 63rd Street. “I think it’s probably in the extreme range of what the machine can do. It’s a pretty tight radius,” says Martinez. “It starts around 830ft (252.98m) radius and ends about 620ft (188.98m) radius. The reason they had to keep it so tight is because of eminent domain and zoning issues.”
He hopes that by the end of 2011, S3 will be completing this eastern bore, and close to completion in the western bore with waterproofing and concreting. Phase One is expected to be finished in 2016.
The TBM was assembled in May 2010, and mined the western bore first Figure 1 – The Second Avenue Subway will be built in four phases; Phase One (inset) is schedule to be operational by 2016 Figure 2 – Phase One TBM excavation started at 92nd Street to take advantage of the Manhattan Schist Utility work along Second Avenue The muck removal system brings material to the launch box, where a bin is used to store it until trucking operations can start from 7am each day
